The development of soft x-ray light sources, such as x-ray lasers and synchrotrons, depends upon the availability of suitable optical components. This is also true for a number of potential applications of x-ray optics, such as x-ray microscopy, lithography, plasma physics diagnostics, medical imaging, lasers, astronomy, etc. Unfortunately, the index of refraction of all materials in the wavelength range below approximately 1000 .ANG. is very close to 1.0, so that the amount of light reflected at normal incidence (90.degree.) onto a single interface (such as a silver film or glass) is less than a few percent for x-ray wavelengths below 300 .ANG..
Since for angles sufficiently close to grazing (i.e. 0.degree.) the reflectivity for metals approaches one, reflective optical elements can be fabricated in the x-ray range. However these "grazing incidence optics" suffer from a number of deficiencies, including severe off-axis abberations and large size. For these reasons, normal incidence optics are highly desirable.
Recently it has become possible to produce multilayer thin film coatings with sufficient perfection to have reasonable reflectivities (&gt;25 percent) in the soft x-ray range of the spectrum (approximately 20-500 .ANG.). This makes possible the use of these multilayer coatings for reflective optics. These multilayer coatings are fabricated to make use of the constructive interference properties of light, so that the light reflected from each interface in the multilayer will add with the correct phase relationship to give a high final reflectivity for the coating. The spacing of the layers in the multilayer can be accurately designed using knowledge of the optical constants of the components, and is optimized for the particular application (i.e. polarizer, normal-incidence mirror, etc.). Because the multilayer must provide constructive interference, in most cases the layers will have a thickness of roughly 1/4 the wavelength of light, e.g. 25 .ANG. for 100 .ANG. light. The total number of layers required, and hence the total multilayer film thickness, will depend upon the application, but will typically be much less than 1 micro-meter (40 micro-inches).
These multilayer coatings must be deposited on smooth, flat substrates to provide support during the fabrication process. Silicon wafers, glass, and sapphire are common substrate materials, although other materials may be used. Typically these substrates are at least 0.5 mm (0.020") thick to provide sufficient strength for the processing steps. All materials strongly absorb soft x-rays. Because of this, any x-rays transmitted through the multilayer will be completely absorbed by the substrate. This makes it very difficult to make laser cavity mirrors or transmission polarizers which require part of the beam to pass completely out of the system. Schemes have been developed to deposit the multilayer on a Si wafer, and then to preferentially etch through a photolithograpically defined mask in the back of the wafer to leave a small region of free standing multilayer without a substrate on the back. SiN membranes created by diffusion or ion implantation, followed by subsequent etching of the Si wafer have also been used to make transmission through the supporting substrate possible. These technologies require the use of Si substrates, which then limits the possible multilayers which can be grown since some multilayer materials are incompatible with Si. Also, other substrates can be much smoother than Si, which is an advantage for obtaining highest performance.